The instant invention deals with graphene-based surface coatings on lead grids for lead-acid batteries to improve the adhesion between the grids and active material pastes, and to reduce the corrosion of the grids. The objective is to improve the performance and life of lead-acid batteries.
Lead-acid batteries (PbA) are one of the most widely used rechargeable batteries in the world, especially for automotive and uninterruptible power supply applications. Traditionally, automotive lead-acid batteries are mostly used for starting, lighting, and ignition (SLI). Such batteries can withstand frequent shallow charging and discharging, but, repeated deep discharges will result in capacity loss and premature failure, as the electrodes disintegrate as a result of mechanical stresses caused by deep cycling.
Additionally, starting batteries kept on continuous float charge tend to have corrosion in the electrodes which will result in premature failure. For some other applications such as UPS, forklifts, etc., lead-acid batteries are designed for deep charge and discharge, but at limited number of cycles. These batteries have low peak currents. Lead-acid batteries have been a relatively mature technology and have been in service for over 100 years.
In recent years, lead-acid batteries have received a lot of attention due to their new potential applications. One of them is in stop-start or micro-hybrid electric vehicles. In such automobiles, the stop-start system automatically shuts down and restarts the internal combustion engine to reduce the amount of time the engine spends idling, thereby reducing fuel consumption and emissions. This is most advantageous for vehicles which spend significant amounts of time waiting at traffic lights or frequently come to a stop in traffic jams. The stop-start function will significantly improve the fuel efficiency and reduce the tailpipe pollution. The traditional lead-acid batteries are attractive for such applications due to their low cost.
Current lead-acid batteries do not meet the performance targets under the cycling conditions of micro-hybrids. There are several major hurdles that need to be overcome. For example, the negative electrode tends to degrade due to the progressive accumulation of PbSO4 under partial state-of-the-charge, high current, and shallow depth-of-discharge.
Other major failure modes are the corrosion of lead grids and delamination of the active material paste from the grids. Both will increase the impedance of the battery and even lose the structure support for the electrode plates. This invention is related to resolving the problems associated with the lead grids.
Lead grids are used as the current collectors and support on which an electrode paste is coated to form a positive or negative plate. For automotive batteries, the positive and negative grids are often designed and manufactured in different forms due to the fact that they are subjected to different electrochemical environments and suffer different types of corrosion and at different levels. The grid surface corrosion is one of the main failure mechanisms for lead-acid batteries.
The corrosion reduces the adhesion between the grid and the active material. When the grid is no longer able to provide structure support and current flow, the battery fails. Therefore, improving the adhesion between the lead grid and paste mixture and reducing corrosion of the grid is one of the key approaches to enhance the performance and extend the life of a lead-acid battery. This is even more important for the stop-start type of applications where frequent, high current, and deep charge and discharge are all needed at different times.
Several methods have been developed to improve the adhesion between lead grid and the active material. For example, a layer of tin, lead-antimony, lead-silver, or lead tin alloy has been coated on the surface of lead-calcium grid to improve the adhesion and protection. Similar surface layers have also been applied by roll-bonding or fusing to the grid.
Chinese patent CN101969143 discloses a method for preparing a nano high-energy maintenance-free lead-acid battery which includes a step of forming superfine glass fiber layers on the surfaces of grids made of a nano ceramic powder and lead metal powder material.
Chinese patent CN201877504 relates to a lead grid consisting of a conducting material layer and a composite material layer. The composite material layer consists of one of lead or lead alloy coating layer, a foam lead layer and an acid-resistant coating layer. The two sides of the conducting material layer are coated with the lead or lead alloy coating layer on which an acid-resistant coating layer is coated. The conducting material layer in the middle of the plate grid serves as a current transmitting passageway so that the resistance is greatly reduced, and the current distribution is more even.
Chinese patent CN10270952 discloses a method for preparing lead-acid battery positive electrode plate that includes the steps of: preparing a positive electrode grid body, conducting electrochemical surface modification of the lead alloy positive grid body, post-treatment of the modified surface of the positive lead alloy grid, and washing and drying of the resulting rare earth modified lead alloy surface of the positive grid.
Chinese patent CN104821402 uses a surface roughening method to improve the adhesion between lead grids and active pastes. The method is mainly characterized by carrying out a plate grid surface roughening treatment, wherein a roughening treatment is performed on the surface of the continuous plate grid framework structure. According to the invention, the bonding force of the punching plate grid and the lead paste can be improved and the method is especially suitable for production of the high-power storage battery punching plate grid.
Chinese patent CN104362301 discloses a preparation method for a carbon coated titanium-based lead dioxide positive plate which is obtained by coating a carbon material on the surface of a metallic titanium mesh with a vapor deposition method.
There are other methods to improve the grid performance in lead-acid batteries. For example, lead-carbon, including lead-graphene and lead-graphite, composites have been tested as possible positive current collectors for lead-acid batteries. It has been shown that neither graphene nor graphite participate in the electrochemical process but they improve corrosion and electrochemical characteristics of both metallic composite materials. No products of interaction of lead with sulfuric acid were formed on the surface of graphene and graphite. Graphene inclusions in lead prevent formation of ready oxide nanocrystals which deteriorate discharge characteristics of positive electrode of lead-acid batteries. Preparation of lead-graphene or lead-graphite composite, however, was performed in molten alkali halides media, thereby increasing the processing complexity and cost.